Nice, I always wanted to make something like this for my school notes but never got around to it. One thing that could be useful is if it would let you write longer paragraphs of text, which could be expanded on-demand. Ex.
# Fruits
## Orange
Oranges were first grown in...
## Banana
Bananas originated in...
Then the mind map would have Fruits -> [Orange, Banana], and you could click on Orange or Banana to expand the full content of the note. This is how I format my notes at least.
Yeah this breaks down if you use anything more than a few words per node, because it has no wrapping. I think even if you had the text blocks wrap at a sane width (20% of screen-width or something) then it would be more better, or expandable/modal blocks like your idea.
